CPA1 Mk III & CLA1 Mk III Instructions
Please read carefully before installing
The VAC CPA1/CLA1 Mk III Preamplifiers are designed for the most exacting applications. The two
models are similar except for the omission of the phonograph preamplifier in the CLA1. All audio
circuits are Class A triode vacuum tube types. In addition, all direct audio signal paths are hard-
wired for best performance.
General Precautions
1.
DO NOT OPEN POWER SUPPLY OR PREAMPLIFIER CHASSIS - THERE ARE NO
USER SERVICEABLE PARTS INSIDE. DO NOT REMOVE TUBE CAGE UNTIL UNIT HAS
BEEN SWITCHED OFF FOR 15 MINUTES. TUBES BECOME HOT IN USE. DO NOT
EXPOSE EITHER CHASSIS TO MOISTURE. DO NOT PLUG INTO AN AC SOURCE
UNTIL ALL CONNECTIONS ARE COMPLETED.
2.
Complete all connections
before
connecting the AC cord to the wall outlet, particularly the
6 pin cable from the power supply to the preamplifier chassis. Similarly, always remove the
AC power cord before detaching the 6 pin DC power cable. Failure to observe this
sequence will result in damage to the power supply.
3.
At turn on, all volume controls should be full counter-clockwise and the mode switch should
be set to MUTE. Allow 1 minute for warm up before selecting STEREO.
4.
For protection of your speakers and power amplifier, always switch preamp to MUTE before
switching the power supply off or on.
5.
Allow the system to sit switched off for at least 10 minutes before disconnecting the preamp
from its power supply.
Operation - Line Stage Section
The line stage can be operated in two basic modes, Active and Passive-Buffered. The active mode
may be of high gain or moderate gain. These three options are selected by a rotary switch on the
top of the chassis on the right side.
Mute the preamp before operating this switch
.
In the
Passive-Buffered mode
the
amplifying
triode (V4) is bypassed, and the controls are fed
directly to a Class A
impedance translating
Class A triode tube (V5). Since the output is buffered,
there are no gross cable interaction problems of the type encountered with passive control centers;
since there is no gain stage, this mode has ‘unity gain’. The VAC Passive-Buffered mode is
capable of driving long cable runs and the preamplifier may be located some distance from the
